CD81: A Novel, Specific and Highly Sensitive Marker in Flow Cytometric Diagnosis of Plasma Cell Dyscrasia

The percent abnormal plasma cells (aPC) as determined by flow cytometry (FC) has been shown to be an independent risk factor for progression from myeloma precursor disease (monoclonal gammopathy of uncertain significance, MGUS; smoldering multiple myeloma, SMM) to multiple myeloma (MM). However, differentiation of aPCs from normal PCs (nPCs) in these patients is challenging. MM cell lines are know to underexpress the tetraspanin proteins (e.g. CD81, CD82) in comparison to nPCs. Although CD81, a nonglycosylated tetraspanin, is robustly expressed on the surface of nPCs, little information is available regarding its expression in the aPCs of MM, SMM and MGUS. In this study we evaluate the expression of CD81 in conjunction with CD19, CD45 and CD56 in bone marrow aPCs and nPCs from patients with MM, SMM and MGUS.

Bone marrow aspirates from 41 patients (9 MGUS, 22 SMM, 7 MM, 3 non-neoplastic with clinical suspicion of MGUS) were analyzed with 8-color multiparametric FC using a panel of antibodies (CD138, CD38, CD19, CD20, CD27, CD28, CD45, CD56, CD81, CD13, CD14, CD16, CD3, CD34 and intracellular kappa & lambda light chains). The pattern of surface antigen and intracellular light chain expression was utilized to determine the percent aPC (defined as monoclonal with aberrant antigen expression) and percent nPC (defined as polyclonal with normal antigen expression). In all cases the pattern of antigen expression was evaluated in the aPCs; additionally, in cases with greater than 5% nPCs (19/41 patients: 8 MGUS, 8 SMM and 3 non-neoplastic) the pattern of antigen expression was evaluated in the nPCs. The ability to detect clonal aPC by evaluation of FC pattern of antigen expression was determined and compared for CD19, CD45, CD56 and CD81. We also examined the sensitivity and specificity of the CD19 and CD81 combination verses the conventional combination of CD19, CD56 and CD45 (Perez-Persona et al, Blood 2007) for the detection of clonal aPC.

CD81 was strongly expressed by nPC (average mean fluorescent intensity (MFI): 11500, standard deviation (SD): 5061, range: 5347–21657) in contrast to aPC with abnormally weak expression (average MFI: 1487, SD: 887, range: 647–4311). CD81 was a highly reliable marker for the detection of clonal PC; with 90% sensitivity and 100% specificity. It was the most specific and second most sensitive marker in our study (Table 1). CD81 was equally sensitive in detection of aPCs in MGUS, SMM and MM. Evaluation of the combined pattern of expression of CD19 and CD81 resulted in 100% sensitivity and 100% specificity for detection of aPC, which is greater than the conventional combination of CD19, CD56 and CD45, yielding 100% sensitivity but 90% specificity, for diagnostic evaluation of aPC.

CD81 is a highly reliable marker in the detection of abnormal plasma cells in MM, SMM and MGUS. The combined approach of CD19 and CD81 is superior to other conventional marker combinations (i.e. CD19, CD45, and CD56) in terms of detection of clonal plasma cells and may replace their use in the clinical evaluation of bone marrow aspirates for plasma cell processes. Furthermore, it should help widening the applicability of minimal residual disease testing in MM.

No relevant conflicts of interest to declare.